Question

Mr. Frank, a former stroke victim who had made a remarkable recovery, suddenly began to have problems reading. He complained of seeing double and also had problems navigating steps. He was unable to move his left eye downward and laterally. Which cranial nerve was the site of lesion? (Right or left?)

Accepted Answer

Identify the symptoms described: inability to move the left eye downward and laterally, double vision (diplopia), and difficulty with spatial navigation (such as steps). Recall the functions of the cranial nerves controlling eye movement: Cranial Nerve III (Oculomotor), Cranial Nerve IV (Trochlear), and Cranial Nerve VI (Abducens). Understand that the inability to move the eye downward and laterally specifically implicates the Trochlear nerve (Cranial Nerve IV), which innervates the superior oblique muscle responsible for downward and lateral eye movement. Determine the side of the lesion by noting that the left eye is affected, so the lesion is on the left Trochlear nerve because each Trochlear nerve controls the superior oblique muscle of the ipsilateral eye. Conclude that the lesion is on the left Cranial Nerve IV (Trochlear nerve), which explains the symptoms of double vision and impaired downward and lateral movement of the left eye.

Key Concepts

Cranial Nerve III (Oculomotor Nerve) Function

Clinical Signs of Oculomotor Nerve Lesion

Lateralization of Cranial Nerve Lesions